* There's a non-zero chunk of the population not near a Walmart or similar.
* It costs time to go there
* It costs vehicle depreciation
So if you were going to Walmart anyway, or walked there with nothing better to do you saved the money.
But if you didn't then let's say the average wage is $25/hour or so and you spent 30 minutes there and back, and fully loaded vehicle depreciation is 50 cents/mile and it's 5 miles away then you spent $17.50 to go there.
I totally realize we could argue all this back and forth but just pointing out that most of the time the "savings" aren't real if you value your time and transport. And yes, I realize most people don't value their time and treat their transport as a sunk cost even if it isn't entirely.
edit: I was assuming going to walmart in person as opposed to online, because that's what most people appear to do but I'm happy to be wrong :-)